textproduct: Rapid City

KEY MESSAGES

- Cooler and wet conditions will persist through the weekend.

- Gradual warmup early next week.

UPDATE

Issued at 1010 PM MDT Thu Sep 17 2026

No big updates to the forecast this evening. As of this writing, widespread showers and isolated thunderstorms are drifting into the area from the south. So far, heaviest radar estimated rainfall totals are in the southern South Dakota plains at around 1 inch to 1.5 inches but actual reporting stations are reporting around 0.50 inches to 1 inch. That being said, those storms that tracked through that area have been convective so it's likely the heavier rainfall missed the stations.

Going forward, these rain and storm showers will keep drifting northward through tonight then shift eastward Friday. As the rain slides eastward, patchy fog is possible. For now, thinking the fog is most possible in the southern SD plains.

Another round of rain and isolated storms is possible Friday night with continued rain chances through the weekend. Forecasted 24 hour rainfall totals for Saturday and Sunday are around 0.50 to 1 inch both days. High-end estimates are upwards of 1.50 inches so locally high rainfall amounts are likely.

DISCUSSION

(This Evening Through Thursday) Issued at 110 PM MDT Thu Sep 17 2026

Water Vapor and upper air models show southwest flow aloft, as ridging sits over the northern and central plains. Upper low currently sits over the west coast, while a shortwave currently sits over WY riding the southwest flow into the Dakotas. 18Z surface analysis shows associated surface low over the CO/WY border. Winds are currently out of the southeast, with temps ranging from around 60 to mid 70s this afternoon. Radar currently has patchy precipitation over the southern half of the CWA as the disturbance moves into the region.

Shortwave will continue to lift into the Dakotas today, bringing widespread precipitation with it. HREF 24 hour probabilities for 0.5" or more QPF show the highest chances (60-95%) over south- central SD. Much of the area could at least see wetting rain through Friday morning. Some areas could see some lightning at times with this precipitation, however severe storms are not anticipated.

Friday will be mostly dry as the shortwave exits the area early morning. Shortwave ridging sets up over the northern plains, but by Saturday the aformentioned upper low shifts over the Rockies bringing cooler temperatures and more potential for widespread precipitation Saturday and Sunday. A bit more uncertainty with QPF, as much will depend on the track of the low as it moves into the northern/central plains, but currently the wettest day looks to be Sunday. Decent chance that some areas will see more wetting rains, with potential for much of the area to see upwards of a half inch Sunday into Monday morning.

By the latter half of monday, upper ridge builds over the plains, bringing a warming trend Monday through much of next week. Temperatures climb back to at or above normal by Tuesday and Wednesday.

AVIATION

(For the 00Z TAFS Through 00Z Friday Evening) Issued At 546 PM MDT Thu Sep 17 2026

Widespread rain showers with isolated thunderstorms will continue to move through the area this evening with MVFR/IFR conditions in heavier showers. Rain showers and thunderstorms will begin to exit the region to the east around 09-12z Friday morning. Low clouds and fog are expected overnight Thursday into Friday morning with IFR/LIFR conditions at KRAP. Low confidence on the fog and low clouds impacting KGCC so removed the lower aviation conditions. Will reassess later and update if needed.

UNR WATCHES/WARNINGS/ADVISORIES

SD...None. WY...None.